The invention relates to a method of coating a luminescent material with a protective layer.
The invention also relates to a luminescent material coated with a protective layer and to a discharge lamp equipped with a luminescent screen comprising such a luminescent material.
A method as mentioned in the opening paragraph is known from EP 0476207 A2. This document describes the coating of a silicate phosphor making use of chemical vapour deposition. Coated luminescent materials show a relatively high stability in the watery slurry that is used in a lamp manufacturing process and an improved maintenance of the light output, when used for instance in the luminescent screen of a low pressure mercury discharge lamp. A drawback of chemical vapour deposition is that the equipment is very expensive. Alternatively coatings can be applied to luminescent materials by means of homogeneous precipitation from a watery solution. Such a process is described for instance in WO 96/05265 and is mainly used to apply metal oxide layers wherein the metal is chosen from the group Y and Al. This latter process is far cheaper than chemical vapour deposition but needs to be carried out at a relatively low pH to prevent the precipitation of large quantities of metal hydroxide. However, many luminescent materials (for instance materials having a silicate host lattice) are sensitive towards hydrolysis at the relatively low pH values at which the homogeneous precipitation is taking place. Additionally it has been found that the optical properties such as reflectivity and quantum efficiency of other luminescent materials that do not or hardly hydrolize in water are adversely affected by a low pH value.
The invention has for its object to coat a luminescent material by means of homogeneous precipitation from a watery solution in which hydrolysis of the luminescent material and degradation of its optical properties are to a large extent prevented.
According to the invention, the luminescent material is dispersed in a watery solution of water soluble polyphosphate, a watery solution of a water soluble salt of a metal M chosen from the group formed by Ca, Sr and Ba is added to the dispersion after which the luminescent material is separated from the watery dispersion and is dried and heated.
The product is a luminescent material coated with a protective layer, which comprises a polyphosphate of general formula (M0.5PO3)n in which metal M is chosen from the group formed by Ca, Sr and Ba.
It has been found that the precipitation of a polyphosphate of a metal M chosen from the group formed by Ca, Sr and Ba using a method according to the invention can very well be carried out at a relatively high value of the pH. As a consequence hydrolysis and degradation of the luminescent material are strongly suppressed.
Good results have been obtained with a method according to the invention, wherein the water soluble polyphosphate is sodium polyphosphate and/or wherein the water soluble salt of the metal M is a nitrate.
Preferably the pH of the watery solution of water soluble polyphosphate is adjusted to be in the range 8-10 before the luminescent material is dispersed in it. It was found that both hydrolysis and degradation of the luminescent material were effectively suppressed.
To effect a dense coating with a thin layer of polyphosphate on the surface of the particles of luminescent material, the dispersion is preferably treated with ultrasonic waves to desagglomerate the luminescent material before the watery solution of the water soluble salt of the metal M is added.
The method was found to be very suitable for coating luminescent materials comprising a silicate host lattice. Good results have been obtained for BaSi2O5 activated with lead and (Ba, Sr)2MgSi2O7 activated with lead. More in particular in case M was chosen as Ca.
A luminescent material coated by means of a method according to the invention was found to be very suitable to be used in a discharge lamp, more in particular a low pressure mercury discharge lamp.